Structural Pathway for Water Permeation through Sodium-Glucose Cotransporters

Abstract

Although water permeation across cell membranes occurs through many transporter proteins, the only permeation mechanism resolved at atomic resolution is that for aquaporins. Recent crystallization of the Vibrio parahaemolyticus sodium-galactose transporter (vSGLT, in the inward facing configuration) allowed investigation of putative water permeation pathways through both vSGLT and the homologous human Na-glucose cotransporter (hSGLT1) using Grand Canonical Monte-Carlo and Molecular Dynamics simulations. For vSGLT, simulations showed the presence of a water-filled pathway which is interrupted in the middle of the protein. In contrast, an hSGLT1 homology model produced a continuous water permeation pathway passing through the sugar binding site and featuring a single constriction zone. Potential of mean force (PMF) calculations indicate that water accessibility to the constriction zone is nearly without cost energetically. However, the constriction zone is associated with an energy barrier of 8±2 Kcal/mol which is modulated by the position of side chain residues located in transmembrane segments 2 and 6 (F101 and A105 in TM2, M283 and L286 in TM6). This value is similar to previously published activation energy values for the passive water permeability of rabbit SGLT1 (9 ± 1 and 5 ± 1 kCal mol-1 (Loo et al., 1996 and 1999)). Comparison with multiple crystallized conformations of a structurally homologous cotransporter (Mhp1) suggested conservation of the constriction zone during a complete cotransport cycle. Electrophysiology coupled with volumetric measurements of hSGLT1-expressing oocytes showed little dependence of cotransporter-mediated water permeability on the extracellular presence of sodium or on the membrane potential applied. This confirms that the cotransporter passive water permeability is independent of conformation. These results provide groundwork for understanding the structural basis of cotransporter water permeability.